1. Field of the Invention
The present invention relates generally to a method for accessing a mobile communication system by a mobile terminal, and in particular, to a method for selecting a system by a mobile terminal.
2. Description of the Related Art
To cope with the rapid development of mobile communication systems and the large increase in the amount of data being serviced in the mobile communication systems, 3rd Generation (3G) mobile communication systems with the necessary throughput for transmitting the increased amount of data at higher speed have been developed. For the 3G mobile communication systems, Europe has adopted asynchronous Wideband-Code Division Multiple Access (W-CDMA) and North America has adopted synchronous Code Division Multiple Access-2000 (CDMA-2000) as wireless access standards. Generally, in the mobile communication systems, mobile terminals communicate with each other via one base station.
With the development of mobile communication technology, the mobile communication systems can support various communication schemes such as Advanced Mobile Phone Service (AMPS), CDMA, Global System for Mobile communication (GSM), General Packet Radio Service (GPRS), W-CDMA, etc. Mobile communication systems can be classified into various systems according to the communication scheme used and frequency band for each communication service provider. Generally, CDMA service provider supplies the information needed by a mobile terminal for system acquisition through Preferred Roaming List (PRL). The PRL is used when the mobile terminal intends to select or maintain a system (or base station), or to perform roaming.
PRL is roughly composed of two tables: one is an acquisition (ACQ) table including acquisition type (system type), band class (frequency band), and channel information, and another is a system search table including System Identifier (SID), Network Identifier (NID), system availability indication (PREF_NEG), geographic information (GEO), Priority (PRI), Acquisition Index, Roaming Indicator, etc.
Generally, if a mobile terminal succeeds in primary channel acquisition using a PRL ACQ table or other channel information in an initial system acquisition process, the mobile terminal receives a synchronous channel message and an overhead message over the primary channel, and determines a channel over which it will receive a service, using channel information provided by a CDMA Channel List Message (CCLM) and its own phone number information. If frequency handoff occurs as a mobile terminal moves while in service, the channel over which the mobile terminal receives the service changes again.
In order to acquire a system within the shortest possible time even though the mobile terminal is powered on after power-off while in service, the mobile terminal manages the last-serviced (or most recently serviced) channel information in a separate table. The table used in this case is called a Most Recently Used (MRU) table.
The mobile terminal stores the MRU table in an Electrically Erasable Programmable Read Only Memory (EEPROM), a non-volatile memory, and upon power-on, reads the information from the MRU table stored in the EEPROM and configures the system scan list using the information.
In addition, if an abnormal situation occurs during the service, the mobile terminal configures the system scan list using MRU table, PRL ACQ table and other channel information.
The system scan list is a list obtained by ordering the systems for which the mobile terminal will attempt acquisition, according to priority. After configuration of the system scan list, the mobile terminal attempts system acquisition in the order of the configured system scan list.
FIG. 1 illustrates a method for configuring a system scan list by a mobile terminal in a general mobile communication system.
Upon power-on, using system information included in the MRU table and the PRL ACQ table the mobile terminal configures the system scan list in the following order.
1. The mobile terminal registers, in system scan list 101, a serving system 110 and systems registered in an MRU table 140.
2. The mobile terminal registers, in system scan list 101, the systems registered in a PRL ACQ table 120.
Referring to FIG. 1, serving system 110 is a system that has last provided a service to the mobile terminal. In most cases, serving system 110 corresponds to the first system (or uppermost system) in MRU table 140. The mobile terminal first registers a serving system, i.e. Digital Cellular Service (DCS) 110 in system scan list 101 (see reference numeral 1), and then registers systems of MRU table 140. Because DCS #738 140a, which is a first system in the MRU table 140, is already registered in system scan list 101, the mobile terminal does not register it in system scan list 101 (see reference numeral 2), and registers DCS #386 140b, which is the next system, in system scan list 101 (see reference numeral 3).
After registering MRU table 140 in system scan list 101, the mobile terminal registers a PRL ACQ table 120 in system scan list 101. For this, the mobile terminal unpacks a PRL file that is stored in the EEPROM after undergoing binary ordering.
After acquiring system information from PRL ACQ table 120 through the PRL file unpacking process, the mobile terminal registers the systems on PRL ACQ table 120 in system scan list 101 in order (see reference numerals 4 to 11). As shown in FIG. 1, the registration from PRL ACQ table 120 into system scan list 101 is performed in order of DCS #29 4, DCS #111 5, DCS #779 7, Personal Communication Service (PCS) #50 8, PCS #100 9, AMPS #29 10, and AMPS #111 11.
For DCS #738 among the systems registered with an index #1 of PRL ACQ table 120, because its system information is already registered in system scan list 101, the mobile terminal does not register it in system scan list 101 (see reference numeral 6).
FIG. 2 illustrates a method for configuring a system scan list when a mobile terminal has lost its system signalling while normally receiving a service in a general mobile communication system.
A mobile terminal using system information included in a MRU table 240 and a PRL ACQ table 220, when it has lost its system signalling, configures a system scan list 202 in the following order.
1. The mobile terminal registers, in system scan list 202, the systems in the same region (or same GEO) as that of the system previously in service, registered in PRL ACQ table 220.
2. The mobile terminal registers, in system scan list 202, the systems registered in MRU table 240.
3. The mobile terminal registers, in system scan list 202, the systems in a different region (or different GEO) from that of the system previously in service, registered in PRL ACQ table 220.
It is assumed in FIG. 2 that the mobile terminal has lost its system signalling while receiving a service as it acquires a system in a region B (or GEO B) 200, e.g. DCS #738, among the systems in PRL ACQ table 220.
In order to reacquire a system, the mobile terminal first registers, in system scan list 202, the systems in the same region as that of the lost system. Therefore, the mobile terminal first registers, in system scan list 202, DCS #738 12 with an index #1 and DCS #779 13 with an index #1, both which are the systems in the region B 200, among the systems in PRL ACQ table 220.
Thereafter, the mobile terminal registers the systems existing in MRU table 240. As for DCS #738 14 which is a first system in MRU table 240, because it is already registered in system scan list 202, the mobile terminal does not repeatedly register it. Therefore, the mobile terminal registers, in system scan list 202, DCS #386 22 which is the next system in MRU table 240 (see reference numeral 15).
Next, the mobile terminal registers, in system scan list 202, the other systems in the different region from that of the lost system, among the systems in PRL ACQ table 220. In the system registration order of the PRL ACQ table 220 shown in FIG. 2, DCS #29 16, DCS #111 17, PCS #50 18, PCS #100 19, AMPS #29 20, and AMPS #111 21 are sequentially registered in system scan list 202.
As described above, in the conventional art, if the powered-off mobile terminal is powered on or encounters an abnormal situation while receiving a service, the mobile terminal newly generates the system scan list according to the situation and attempts system acquisition using the new system scan list. Therefore, every time the mobile terminal newly generates the system scan list or registers PRL ACQ table information, the mobile terminal needs to read the binary-ordered PRL information from the EEPROM and register the PRL information in the system scan list through an unpacking process. In addition, when the developer desires to change the search order of the system scan list, he/she should know all processes for configuring the system scan list, making it difficult to change the search order.